Actually its all people of colour that can be called that and its mainly about being a traitor to your cultural roots rather than trying somehow to appease white people, who generally need no appeasement in the first place. Gammon refers to the colour of the face when a person normally white starts frothing on about something mad, like for example a chap from Yorkshire with two cars on finance and house mortgaged up to the hilt banging on about how the mega rich should not be taxed more.Not similar at all. Coconut is about accusing a black person of trying to ingratiate themselves with white people. It is specifically about race. Gammon is about accusing a middle aged right winger of getting worked up about trans rights.
